Differential mechanisms are included in vehicle transmissions to transmit torque from a transmission output to opposing axle shafts allowing right and left wheels to rotate at different speeds, particularly important when negotiating a turn. While performing generally the same function, differential mechanisms have different dimensional requirements for rear wheel and front wheel drive vehicles. Specifically, differentials intended for use on front wheel drive vehicles require a beveled, and even annular shape in order to compensate for both the smaller packaging area available and to account for the steering characteristics of the front wheels of the vehicle. Rear wheel drive vehicles are typically larger and require more torque production than front wheel drive vehicles and include sport utility, pick-ups, and even heavy duty vehicles. Therefore, the rear wheel drive differential is typically dimensioned larger with heavier gauge steel than is a front wheel drive differential. Iron castings presently used to form the housing of a differential for a rear wheel drive vehicle requires a large number of machining operations to produce finished parts having the dimensions necessary to provide adequate tolerances to support the gears disposed within the housing. Dimensional accuracy is important to produce a vibration free motion of the differential in order to produce a smooth vehicle operation.
Given the high torque requirements that is typical of the rear wheel drive vehicle, it is believed that the differential housing should include a more dimensionally stable and durable configuration than what is required of a front wheel drive differential housing. Furthermore, many of the manufacturing drawbacks of cast differential housings have resulted in excessive cost of a typical vehicle transmission. U.S. Pat. Nos. 6,045,479, 6,061,907, 6,176,152, and 6,379,277 each disclose a method of forming a differential housing using a cold flow-forming process. While the housing formed by the process disclosed in these prior references are believed to be effective for use in a front wheel drive transmission, it is believed that the housings will not be as effective for use in a rear wheel drive vehicle. However, the dimensional improvements produced by the cold flow-forming process that enable sheet steel to be used to form the differential housing can also be used to form a differential housing for a rear wheel drive differential. Therefore, the cold flow-forming process disclosed in these prior art patents are included by reference herein.
Furthermore, improvements made to the cold, flow forming process and performance deficiencies of the prior art patents would also be desirable. For example, the prior art housings have not proven to withstand required durability testing due to the location of the shaft holding the pinion gears inside the housing. Specifically, sandwiching the shaft between two housing portions does not provide the durability required of a rear wheel drive differential.